Fast-neutron nuclear reactors cooled by liquid metal generally comprise a main vessel of large dimensions enclosing the liquid cooling metal, most often sodium, in which the reactor core is immersed. Inside the main vessel there are arranged internal structures supporting the core and enabling the sodium circulating inside the vessel to be channelled and the internal volume of the vessel to be separated into different parts where the liquid sodium is at different temperatures.
One part of these structures, which comprises an ogival toric shoulder surmounted by a substantially cylindrical sleeve, forms an internal shell separating the internal volume of the main vessel above the core support into a hot collector situated inside the internal shell and a cold collector situated outside the internal shell.
The components of the reactor, such as the pumps for circulating the liquid sodium and the intermediate heat exchangers in the case of an integrated type nuclear reactor, are immersed in the liquid sodium filling the main vessel and pass with their bottom part through the internal shell in the region of the shoulder.
The top part of main vessel is closed by a plate which also supports the vessel and the components. The liquid metal in the main vessel has an upper free level above which there is an inert gas such as argon, inside a space situated underneath the closure plate.
This upper free level of the liquid sodium is essentially variable during operation of the reactor; it is in fact capable of moving between two well-defined positions inside the internal shell which will be referred to below as the high level and low level.
The level is, on the other hand, constant inside the main vessel of the reactor owing to the presence of a run-off assembly.
The top end of the internal shell is located above the high level of the sodium, such that this shell permanently ensures separation of the internal volume of the main vessel into two zones.
The top part of this shell is therefore subjected to a high axial temperature gradient due to the presence of the liquid sodium/inert gas interface, both during permanent-state operation of the reactor and during transient operational states which are accompanied by variations in the free level of the liquid sodium.
This axial temperature gradient is accompanied by thermomechanical stresses in the top part of the internal shell, such that it is necessary to monitor the operating performance of this shell and avoid operational states accompanied by excessively rapid displacements of the free level. In particular, when the reactor is started up, it is necessary to limit the rise in temperature of the sodium, and this increases the duration of these start-up operations.
The inert gas located above the liquid sodium is in fact at a temperature which constantly remains well below the temperature of the liquid sodium. The emersed part of the internal shell is therefore much colder than the immersed part.
Furthermore, in order to limit the effect of any stresses of seismic origin on the internal shell, it is necessary to fix on the top part of this shell a reinforcing structure which generally consists of an annular part welded to the top end of the shell. The presence of this reinforcement accentuates the thermal inertia phenomena and increases the axial temperature gradient during transient states.
FR-A-2,532,629 and GB-A-1,431,371 describe devices for reducing the thermal stresses in the wall of a vessel such as the external vessel of a nuclear reactor cooled by liquid sodium, consisting of an annular enclosure which is supplied with hot liquid metal either via its bottom part or via its top part, by annexed means. In this way, the part of the wall of the vessel in contact with the hot liquid metal introduced into the enclosure is kept at a fixed temperature whatever the variations in the level of the liquid metal inside the vessel.
Such a device, which requires annexed means for supplying the annular enclosure with hot liquid metal, cannot be applied to the case of the internal shell of a fast-neutron nuclear reactor and is not capable of limiting the thermal stresses in the wall, during all stages of use of the reactor vessel. In fact, protection is not ensured when the reactor is shut down.